1. Field of the Invention
The present invention relates to skyline logging carriage and systems used in yarding logs.
2. Description of the Prior Art
In one known skyline yarding system, an overhead carriage is suspended for travel along a fixed skyline between a cutting site in the woods where the timber is felled and bucked into logs, and a log landing or collection point where the cut logs are loaded, typically onto trucks, for transportation to a sawmill. The skyline carriage is pulled along the skyline from the woods to the landing by an in-haul cable wrapped about a carriage drum at one end and, at the other end about a powered winch drum on a yarding machine at the landing. The carriage is pulled back to the woods by a haulback cable wrapped on a second carriage drum at one end and about a second powered winch drum on the yarder at the opposite end. The haulback cable passes from the carriage rearwardly to a tail block in the woods and then forwardly to its winch drum. A brake is applied to the carriage drums to enable carriage travel. A drop line is suspended from a third carriage drum interconnected to the other two such drums, and choker cables are attached to the lower end of the drop line for connecting logs to the drop line so that they can be carried or skidded from the woods to the landing with the carriage. This is a conventional three-drum skyline yarding system.
Where the landing is uphill from the cutting site, a so-called "shotgun" or gravity yarding system can be used. In this system the haulback line and its drum can be eliminated because gravity is used to propel the carriage along the skyline from the landing back to the woods.
Heretofore gravity skyline yarding could not be successfully carried out using a conventional three-drum carriage as described because the positioning of the carriage drums interfered with the desired rigging of the carriage drum cables for this purpose. Furthermore, such carriages in the past have been equipped with cable clamps effective to hold the carriage against movement along the skyline in only a forward direction, whereas in gravity yarding a clamp must also be capable of holding the carriage against movement in the opposite, downhill direction. Consequently, in the past special shotgun carriages have been used for gravity yarding. Such shotgun carriages have relied on gravity for lowering the dropline for attachment to and releasing a load, and gravity is sometimes unreliable for this purpose.
However, one desirable feature of a gravity skyline yarding system is that it is much easier and faster to set up, as will be apparent from a comparison of FIGS. 1 and 2. Since there is no haulback line, no tail blocks are required in the woods for the haulback line. Only the skyline needs to be anchored in the woods.
Skyline carriages are of many types and varieties. Typical skyline carriages of the prior art are shown in, for example, U.S. Pat. Nos. 3,776,390 (three-drum); 3,844,419 (no carriage drums); Re. 27,621; 3,083,839; 3,712,478; 3,718,262; 3,531,000; 3,863,774; and 3,247,933. None of them, however, are suitable for use in both gravity yarding and conventional three-drum yarding as described. Accordingly, there is a need for a skyline carriage suitable for use both in conventional three-drum skyline yarding and in gravity skyline yarding.